It is known to incorporate a GPS (global positioning system) positioning receiver in a mobile telephony handset. One application for such a handset is to allow wireless transmission of handset location information to emergency services, but other applications are envisaged to provide so-called location-based services such as displaying location to the handset user and providing directions to identified destinations. Such functions require rapid satellite signal acquisition and position calculation, leading to receiver complexity and power consumption levels not readily acceptable for a mobile telephony handset.
Satellite-based universal ranging systems which have been proposed or currently exist (such as GPS) transmit ranging signals which are code-modulated in order to spread the signal bandwidth. The receiver for these signals operates to align a replica of the coding sequence with a respective signal received at the antenna in order to achieve correlation and, therefore, to be able to track the signal and to analyse or demodulate the signal. For the civilian (c/a code) GPS signals, a pseudo-random code with a repetition rate of 1 millisecond is used, the code being constituted by 1023 chips (or bits). The same code is repeated at the end of each code sequence. A receiver with no knowledge of time, or of time accurate to better than 1 millisecond, must search all possible code offsets compared to its internally generated code replica until it detects the incoming signal by measuring the power or amplitude of the resulting correlation. Since detection by correlation requires the code replica to be within a 1 half-chip phase of the incoming signal, a conventional GPS receiver must search all possible code offsets, i.e. every possible half-chip phase, of which there are 2046, in order to find the signal.
Accordingly, to achieve rapid signal acquisition in varying signal reception conditions, it is generally necessary to include a bank of 2,046 parallel correlators. Using a correlator bank with fewer correlator channels leads to a lengthy, serial acquisition process which, depending on the limits imposed upon the correlator architecture by handset design constraints, may take several minutes. Performance of this low level is clearly not fit for position-fixing in an emergency situation, nor in some envisaged location-based services.
If the time of arrival of signals at the receiver antenna can be predicted to better than 1 millisecond uncertainty, and if an approximate position is known (e.g. to plus or minus 20 km) together with the positions of the satellites, the time period (or “window”) over which the correlators must search for each transmitted signal can be narrowed, with the result that the search can be performed more quickly or can be achieved with fewer parallel correlators, or a combination of both. Some mobile telephony networks have arrangements for delivering precise time via a base station to the mobile unit in order to achieve this. However, in other networks, this is either not possible, or not achievable without significant investment in additional network elements.
A mobile telephony handset for use according to the GSM standard includes a temperature-compensated crystal oscillator (TCXO) which is typically accurate to one part per million and is phased-locked to an oven-controlled crystal oscillator (OCXO) in the base station during a phone call. However, use of the TCXO as a time reference for position fixing is of limited use not only because of the infrequent locking to the base station oscillator, but also because the TCXO is switched off, along with much of the rest of the telephony circuits, at times other than during a call in order to save battery energy. During these quiescent periods, a comparatively low-accuracy low-power oscillator is used as a clock source. Despite the fact that the TCXO may be activated every few seconds to search for an incoming call, the time-keeping is too poor significantly to help the GPS signal acquisition process.
It is an object of the present invention to improve the performance of a position-fixing receiver in a mobile battery-powered handset in terms of processing speed, power consumption, or receiver complexity, or a combination of these parameters.